I see. I realised the problem is mostly related to the VPN usage. When I open VPN and with just one Youtube video tab open, memory usage goes up to 1.5 GB. This is huge. And this problem has not been solved for so long.

Not exactly "out of memory" for me... but for sure some memory behavior, that I would not call "expected".
When watching streams on twitch, using html5 player, opera really starts occupying my ram.

They are different Opera versions, he's talking about Stable and you are talking about Beta, despite seems the same problem, versions are different, so please search if another discussion with the same problem exist for Beta or create another discussion for that problem.